Rare blue moon appears Friday

Blue moon in Gaza City

Mohammed Abed / AFP/Getty Images

A full moon is seen from Gaza City on Aug. 31, 2012. According to NASA, this is the second time in this month that a full moon is seen -- the first was Aug. 1 to 2. This phenomenon, which is referred to as the blue moon, happens every two and a half years on average.

A full moon is seen from Gaza City on Aug. 31, 2012. According to NASA, this is the second time in this month that a full moon is seen -- the first was Aug. 1 to 2. This phenomenon, which is referred to as the blue moon, happens every two and a half years on average. (Mohammed Abed / AFP/Getty Images)

The last time a blue moon happened was on New Year's Eve 2009. The next blue moon will occur on July 31, 2015, according to moongiant.com. The phrase "blue moon" has been around for more than 400 years, writes Philip Hiscock, a folklore expert at the Memorial University of Newfoundland, in an exhaustive piece on the origin and uses of the term "blue moon." Read more: Tonight's blue moon won't be seen again until 2015